Dear student,
The reaction of Carbon with oxygen to give carbon dioxide is given as:
C + O2 CO2
In the reaction, the stoichiometric coefficient (related to mass) of both the reactant is 1. Hence, one mole of carbon and one mole of O2 gas are reacting. The stoichiometric coefficient of product (which is CO2 ) is also 1. Therefore, 1 mole of carbon and 1 mole of oxygen reacts to give 1 mole of CO2.
Regards!!